2) Cardiac imaging and functional analysis is the largest single nuclear medical imaging application and represents the area of greatest unmet need in the current technology. Physicians require information on the anatomy and function of the heart in order to diagnose, prescribe treatment, and monitor results of intervention.
3) First-Pass Radionuclide Angiography (RNA) provides the clinician with patient information for improved patient management that is either difficult and/or costly to obtain using other technologies. First-Pass RNA procedures have substantial underutilized potential to be used as early diagnostics for coronary artery disease and myocardial infarctions. The speed and ease of administration of the First-Pass RNA diagnostic procedures allow for new use environments such as the Emergency Room environment or in outpatient cardiology clinics, First-Pass RNA can provide unique dynamic information about cardiac function, such as regional ventricular wall motion, ventricular ejection fraction, pulmonary transit-time, contractility, and shunt quantitation.
4) Currently single gamma (non-PET) nuclear medicine cardiac imaging (both tomographic SPECT procedure and planar imaging procedures) is performed with standard nuclear medicine cameras. These procedures are either performed with standard gamma cameras or with newly introduced dedicated fast rate gamma cameras (from Digirad, GVI). Both types suffer from rather poor sensitivity to detect heart abnormalities located in the further part of the heart, away from the chest wall, hence far from the closest possible approach of the camera surface. There are two contributing factors to this limited sensitivity performance related to the basic physics principles of single gamma imaging: (1) spatial resolution that decreases with distance between the heart sector and the detector head as a result of the collimator, and (2) the dominating effect of gamma ray absorption and scattering in the heart tissue and other body organs on its way towards the detector as a consequence of tissue absorption. A single compact dedicated cardiac detector allows placement of the detector directly against the chest wall and thus improves spatial resolution by minimizing the distance to heart. But this will not help with the absorption issue.
5) In the SPECT case, during the tomographic acquisition of series of images from different detector head positions around the patient torso, each sector of the heart has better visibility from some directions of closest approach to that sector (with the exception of the inner sector of the heart equally distant from all directions). In addition, computer algorithm-based modeling of the absorption and spatial resolution effects and subsequent correction of the collected data is improving the quality of reconstructed 3D data. However, the situation is diametrically different during planar imaging, and especially during the dynamic first-pass procedure, when imaging is performed from only one pre-selected imaging direction (one view) and only limited post-acquisition corrections are possible.
6) In addition to first-pass imaging, a practical and most useful imaging cardiac system should be also able to perform other imaging procedures. Some imaging procedures that will benefit from the proposed improvement in the imager include: (1) 1st pass studies, (2) planar, EKG-gated (and non-gated) bloodpool (MUGA) studies, which encompasses extensive phase analysis of the wall motion during heart cycle to appreciate potential damage to the heart muscle, (3) planar EKG-gated (and non-gated) perfusion studies, (4) planar hot spot imaging, and (5) limited positron detection via planar acquisition.
7) Planar gated (MUGA) studies are used to evaluate ventricular wall motion, and elucidate an ejection fraction based on the systolic and diastolic filling (of blood) within the ventricle. Planar perfusion studies are used to evaluate blood flow to the myocardium (left ventricle) and are quantified by established algorithms. Hot spot imaging involves detection only in areas of unique radiotracer uptake, which are higher than background. Although not widely accepted yet, positron imaging of the heart capitalizes on the ability to image metabolism and blood flow within the heart. One of the implementations of the proposed system will have the capability to perform positron imaging studies using the same detector heads but with removed collimators and operating in a coincidence mode. However, this additional function is not at the core of the present invention, which is focusing solely on improvement of single gamma imaging capability.
8) Tc-99m is the most popular label used in nuclear medicine and is in the energy range (140 keV). The major problem associated with the single-view planar (such as firstpass) cardiac imaging procedure is that the characteristic gamma radiation from Tc-99m undergoes substantial absorption when traversing tissues such as heart muscle tissue. As a result for Tc-99m, the gamma ray flux, and the associated imaging signal in the gamma camera, coming from the region of the heart away from the detector is much more attenuated than the gamma rays originating in the front part of the heart. This signal sensitivity asymmetry, can produce an effect equivalent to a less pronounced cardiovascular flow at the back side of heart, and, therefore, provides less diagnostic power as to the quality of the cardiovascular flow in that region. On a statistical basis, this asymmetry effect results in a less pronounced separation between the healthy individuals and people who have cardiovascular disease.
9) Alternative imaging labels with higher-energy gamma emissions undergoing lower absorption in the heart tissue, such as I-131 and In-113m, have been used in the past with success, but are not used in today's clinical environment, are not being manufactured in volume, and produce higher patient doses. Subject of our invention is a partial but substantial remedy to the theoretically expected and clinically observed limitation in the detection of Tc-99m gamma emission from the patient's heart which is adversely impacting the diagnostic quality of all single gamma imaging modalities but especially of the dynamic imaging of a kind performed during the firstpass procedure.
10) The dual-head cardiac imaging system proposed herein will have the following novel features to improve cardiac imaging and functional analysis:                (1) two identical compact light-weight gamma imaging detector heads, mounted on a gantry, will be precisely mechanically co-registered to each other at 180 degrees (placed opposite to each other), placed on both sides of the patient torso and fixed relative to each other, with the patient's heart encompassed by the resulting active field of view,        (2) Two high precision specially designed and produced parallel hole collimators (made out of lead, tungsten, lead alloy, tungsten alloy, or mixtures of lead or tungsten powder with filling materials such as epoxy) will be precisely mechanically aligned, and the alignment will be checked using special QA procedure with line or point Co-57 radioactive sources before each patient scan,        (3) Two individually produced time series of dynamic cardiac images in both detector heads will be obtained using the same start time and time clock, and will be processed as one set of time correlated images by special imaging algorithms involving multiplication of pre-processed co-registered images from both imagers for each time frame.        